The present invention, in some embodiments thereof, relates to material science, and more particularly, but not exclusively, to compositions and methods for reducing the friction coefficient of hydrogels or of composite materials comprising a hydrogel, and to uses of such low friction hydrogels or composite materials containing same.
Lubrication in aqueous media presents a critical challenge in modern material science. Lubrication in aqueous and biological media is often problematic as water on its own is not a good lubricant, while surfaces or surface coatings in water frequently exhibit quite high friction coefficients, higher than 0.08, especially at high loads/pressures. The problem is more evident when extremely low friction is required, particularly at high pressures of up to 100 atmospheres or more, and at low sliding velocities.
Various attempts to provide low friction conditions in aqueous media, and particularly under physiological conditions for treating, inter alia, joint dysfunction, have been made.
Vecchio et al. [Rheumatology (Oxford), 1999, 38(10), pp. 1020-1021] describe the injection of dipalmitoylphosphatidylcholine (DPPC) lipid surfactant solutions in propylene glycol into joints in an attempt to provide a treatment for osteoarthritis.
U.S. Pat. No. 6,800,298 describes a lubricating composition (i.e. a lubricant) comprising dextran-based hydrogel with lipids.
Liposomes are vesicles whose membranes in most cases are based on phospholipid bilayers. They are generally biocompatible and, when modified with other molecules, are widely used in clinical applications, primarily as drug delivery vehicles, as well as in gene therapy and for diagnostic imaging.
WO 2008/038292 discloses, inter alia, multilamellar vesicles or liposomes (MLVs) of several phospholipids above their liquid-crystalline-phase to gel-phase transition temperature (Tm) as possible boundary lubricants in the articular cartilage environment.
WO 2011/158237, by some of the present inventors, discloses, inter alia, a method for lowering the friction coefficient of surfaces, which is effected by applying gel-phase liposomes onto surfaces to form a boundary lubricant layer, wherein the temperature of the surface at the time of lubrication is below the phase transition temperature (Tm) of the liposomes. The described method is suitable for lubricating biological and non-biological surfaces, including the surfaces of a biological tissue in a mammalian subject, e.g., for treating joint dysfunction.
Further studies on surface lubrication by liposomes are described in, for example, Klein et al., Faraday Discuss., 1994, 98, p. 173-188; Goldberg et al., Advanced Materials, 2011, 23(23), p. 3517-3521; Goldberg et al., Chemistry and Physics of Lipids, 2012, 165, p. 374-381; and Goldberg et al., Biophys. J., 2011, 100(10), p. 2403-2411.
A hydrogel is composed of a three-dimensional fibrous network containing up to 99.9% water. Swelling, or uptake of water, is made possible due to the hydrophilic groups attached to the polymer backbone of such hydrogels. The polymer strands may be chemically crosslinked to various extents between groups in the backbone or side-chains, giving rise to a variety of mechanical properties of the hydrogel. Hydrogels can be biocompatible and based on their synthetic or natural occurring polymeric components, can be biodegradable (e.g., by enzymes) or non-biodegradable. These characteristics gave rise to a great interest from the biomaterial field developing many bio-gels based applications including hydrogel-based scaffolds for tissue engineering applications. Among them are the calcium alginate microcapsules [Lim et al., Science, 1980, 210(4472), p. 908-910], alginate hydrogel for myocardial repair [Ruvinov et al., Biomaterials, 2011, 32(2), p. 565-578], and polyethylene glycol hydrogels for neural tissue [Mahoney et al., Biomaterials, 2006, 27(10), p. 2265-2274].
Poly-HEMA-based hydrogel is biocompatible and its water content could reach more than 70%, resembling that of cartilage. Its Young's modulus may be 1 MPa, depending on the exact hydrogel composition, also similar to that of cartilage. Hence, hydrogels based on polymeric constituents such as poly(2-hydroxylethylmethacrylate) (pHEMA) have been investigated for use as synthetic cartilage replacement substance [Petrtyl et al., Acta of Bioengineering and Biomechanics, 12 (3), 2010].
In a study on the tribological properties of pHEMA-based hydrogels for use in artificial cartilage [Bavaresco et al., Wear, 2008, 265 (3-4), p. 269-277], the friction coefficient and wear as a function of different crosslinking densities, crosslinking agents, sliding speed and contact pressures were investigated.
Freeman et al. [Wear, 2000, 241(2), p. 129-135] studied the tribological behavior of pHEMA surface to a stainless steel ball as a function of the load, lubrication, crosslinking density and the degree of the hydrogel hydration.
Gong et al. [, Soft Matter, 2006. 2 (7), p. 544-552] reported the effect of adding surfactants to the water medium on the friction coefficient of a negatively charged polyelectrolyte hydrogel in a parallel-plate rheometer configuration up to low pressures around 1 atm.
Gulsen et al. [Current Eye Research, 2005, 30, p. 1071-1080] teach contact lens compositions with drug delivery capabilities, and specifically teach dispersing exceptionally small dimyristoylphosphatidylcholine (DMPC) SUV liposomes (less than 50 nm or 80 nm in diameter) in poly-2-hydroxyethyl methacrylate (p-HEMA) hydrogels, which are a common contact lens material. The results of this study show that the p-HEMA gels loaded with exceptionally small liposomes are transparent and that these gels release drugs for a period of about 8 days.
DiTizio et al. [Biomaterials, 1998, 19, p. 1877-1884] teach a liposomal soft hydrogel system that reduces bacterial adhesion to silicone catheter material.
Nagarsenker et al. [International Journal of Pharmaceutics, 1999, 190, p. 63-71] teach liposomes dispersed in soft polycarbophil gel.
Mourtas et al. [Langmuir, 2009, 25(15), p. 8480-8488] teach rheological properties of complex soft and semi-solid hydrogels containing different amounts of liposomes and/or cyclodextrin.
Kang et al. [Journal of Drug Targeting, 2010; 18 (8), p. 637-644] teach cationic liposomes composed from less than 50% phosphatidylcholine lipids, dispersed in a soft thermosensitive gel.
Mechanical properties of layers of stable liposomes attached onto solid surfaces, including surfaces of hydrogels, were studied by atomic force microscopy (AFM) force measurements [Brochu, Ph.D. Thesis in the Université de Sherbrooke, Canada, 2008, Id.: 50177338].
Additional prior-art documents include U.S. Patent Application Publication Nos. 20040171740, 20060270781 and 20110293699, and U.S. Pat. Nos. 7,638,137 and 8,273,366.